The present invention relates to sandwich panels for attenuating acoustic energy. The invention relates more particularly to such panels designed for use as acoustic liners or splitters for suppressing jet mixing noise and/or turbomachinery noise, such as in a fan duct of a turbofan aircraft engine.
The noise generated by aircraft engines can be a nuisance to passengers and to people on the ground in the vicinity of airports. Many governments as well as airports and other noise-controlled areas impose strict limits on the level of noise that aircraft are permitted to generate. Generally, in order to meet such regulations, various types of noise suppression devices must be used for suppressing the noise generated by the aircraft engines. For example, in turbofan or turbojet engines it is common to line at least portions of the engine nacelle and/or nozzle duct with acoustic panels for suppressing noise. Such acoustic panels in some cases are designed to suppress noise generated by sources on either side of the panel.
A prior acoustic panel for such applications is formed by a plate or septum having a honeycomb layer bonded to each of the opposite sides of the septum. Each honeycomb layer is covered at its outer surface by a perforated metal plate. Exemplary acoustic panels of this type are described, for example, in U.S. Pat. Nos. 4,265,955 and 4,257,998.
The present invention seeks to provide an acoustic panel providing at least as good attenuation as the above-mentioned type of panel with the least possible weight and performance penalties when used in an aircraft engine or similar application.
The invention provides an acoustic sandwich panel that achieves substantially the same jet noise suppression as the known panel described above, but is significantly thinner and lighter in weight than the known panel. This is accomplished, in accordance with one preferred embodiment of the invention, by constructing the core of the panel from a porous material such as honeycomb without any septum. The porous core can have a substantially smaller thickness than the combined thickness of the two honeycomb layers and septum in the known panel. A face sheet is attached to each of the opposite faces of the core. The face sheets at their outer surfaces include a layer of fibrous cloth such as metal felt or woven wire. In a particularly preferred embodiment, each face sheet comprises a perforated metal plate bonded to a sheet of metal cloth. The perforated plates are attached to the core and the metal cloth sheets form the outer surfaces of the acoustic panel. The perforated plates provide structural rigidity to the panel, and preferably have a relatively large open area so as to have a relatively small acoustic effect compared to the metal cloth sheets. Viscous losses through the metal cloth sheets provide dissipation of the acoustic energy.
In attempts prior to the present invention to construct a no-septum acoustic panel, the panel was formed by a honeycomb core with perforated metal plates attached to the opposite sides of the core so as to form the outer surfaces of the panel. Tests performed on such no-septum panels showed that the attenuation performance was worse than the conventional panel with septum.
The development of the present invention ran counter to the accepted wisdom that no-septum acoustic panels were disadvantageous from the standpoint of acoustic attenuation performance. It was discovered that by including the outer layers of metal cloth, the acoustic attenuation of the panel can be essentially the same as that of the conventional panel having a septum. However, the panel""s thickness and weight can be substantially less than that of the conventional panel. When used in applications in which flow occurs on both sides of the panel such as in a turbofan engine fan duct, the panel of the invention enables substantial reduction in the blockage presented by the panel, which is beneficial to the aerodynamic performance of the engine. The lower weight of the panel is also desirable in aircraft applications.
The invention also encompasses apparatus and methods for suppressing noise in a nozzle. In accordance with the invention, suppressing noise in a nozzle is accomplished by disposing at least one splitter in the duct of the nozzle such that flow occurs along both sides of the splitter. The splitter comprises a core layer having opposite faces, the core layer being a porous material, and a pair of face sheets attached to the opposite faces of the core layer so as to sandwich the core layer therebetween, each face sheet including at least a sheet of fibrous cloth defining an outer surface of the face sheet. In a preferred embodiment of the invention, a plurality of such splitters are disposed in the nozzle duct, the splitters being oriented generally radially and spaced apart circumferentially in the duct.